Calculate the energy cost to heat water. Enter gallons, temperature rise, and heater efficiency to find kWh or therms and total cost.
Water heating is the second-largest energy expense in most homes, accounting for 14–18% of the average utility bill. Whether you have an electric tank, gas tank, tankless, or heat pump water heater, knowing the energy required to heat your water helps you budget accurately and evaluate efficiency upgrades.
The fundamental physics are straightforward: it takes 8.34 BTU to raise one gallon of water by one degree Fahrenheit. Converting that to electricity (kWh) or natural gas (therms) and dividing by your heater's efficiency gives you the actual energy consumed. This calculator handles the conversion automatically — just enter your daily hot water usage, the temperature rise from cold inlet to hot outlet, and your heater's efficiency rating.
The results show energy consumption in kWh (for electric) or therms (for gas) and the associated dollar cost. Use this to compare fuel types, evaluate heat pump water heaters, or estimate savings from reducing hot water demand through low-flow fixtures.
Water heating costs are hidden in your overall energy bill. This calculator isolates the heating component so you can evaluate efficiency upgrades, compare fuel types, and quantify savings from demand reduction. Having accurate metrics readily available streamlines utility bill analysis, budget forecasting, and investment planning for energy efficiency projects and renewable energy installations.
Energy (kWh) = Gallons × 8.34 × ΔT / (3,412 × Efficiency) Energy (therms) = Gallons × 8.34 × ΔT / (100,000 × Efficiency)
Result: $4.62/day
Energy = 64 × 8.34 × 70 / (3,412 × 0.95) = 11.53 kWh/day. At $0.13/kWh, daily cost = 11.53 × $0.13 = $1.50/day, or about $547/year. This represents the energy portion only.
Water has a high specific heat capacity — it takes 1 BTU to raise 1 pound of water by 1°F, or 8.34 BTU per gallon per degree. This makes water heating energy-intensive and explains why it's such a large share of household energy use.
Standard electric tanks are cheap to buy but expensive to run. Gas tanks are cheaper to operate but less efficient. Tankless models eliminate standby losses but have higher upfront costs. Heat pump water heaters are the most efficient electric option, using ambient air to heat water at 2–3x the efficiency of resistance heating.
Before upgrading your water heater, reduce demand. Low-flow showerheads save the most hot water, followed by efficient clothes washers and shorter showers. Every gallon of hot water you eliminate saves both water cost and heating energy.
The typical temperature rise is 60–80°F. If your incoming cold water is 50°F and your heater is set to 120°F, the rise is 70°F. Colder climates have lower inlet temperatures and higher rises.
The energy factor is a measure of water heater efficiency. Standard electric tanks have an EF of 0.90–0.95. Gas tanks are 0.58–0.70. Heat pumps reach 2.0–3.5. Higher is better.
Gas is usually cheaper per BTU despite lower efficiency. However, heat pump water heaters can flip this — their high efficiency makes electric cheaper than gas in many markets.
Use low-flow showerheads and faucets, lower the thermostat to 120°F, insulate pipes and tank, fix leaks, and consider a heat pump or solar water heater for the biggest savings. Consulting relevant industry guidelines or professional resources can provide additional context tailored to your specific circumstances and constraints.
The efficiency input partially accounts for standby losses in tank-type heaters. Tankless models have no standby losses. For precise analysis, use the energy factor provided by the manufacturer.
A therm is a unit of heat energy equal to 100,000 BTU. It is the standard billing unit for natural gas. One therm is roughly equivalent to 29.3 kWh of electricity.